1995A&A...294..443T

Ionization structure of the Cygnus X-3 binary system is simulated on the basis of the stellar wind model of the luminosity modulation and the X-ray spectra observed with GINGA satellite. Emergent spectra are compared with observations to derive constraints on the companion mass and metallicity of the stellar wind. The iron edge absorption energy suggests that the companion is fairly massive; M=7⁺³_–2M_☉. Combining the estimated companion mass with the column density of incompletely ionized iron atoms, we find that iron in the stellar wind is somewhat depleted compared with the cosmic abundance; 0.1∼0.5 times the cosmic abundance. The phase-to-phase spectral variation observed with GINGA is well reproduced if the stellar wind gas is enriched with either C, N, or O. These results strongly support the recent infrared observation that the companion of Cygnus X-3 has the feature characteristic to N-rich Wolf-Rayet stars. The inferred iron depletion may be suggestive to a binary evolution and the nature of Wolf-Rayet stars. A modest interpretation is that the companion of the Cygnus X-3 is classified to an early WN star (WNE) and the metal deficit is due to the galactic abundance gradient or the local abundance anomaly.